The laser has achieved widespread use in many applications. Particularly, the semiconductor laser is extensively employed in optical communication systems as a means for transmitting modulated light signals via fiber optic cables or similar devices.
The diode laser employs a semiconductor where the electron current flowing across a junction between p and n type material produces extra electrons in the conduction band. These electrons radiate upon their making a transistion back to the valence band or to a lower energy state. If the junction current is large enough, a population inversion occurs and lasing is provided. To use this effect, a semiconductor crystal is polished with two parallel faces perpendicular to the junction plane. The amplified waves may then propogate across the plane of the junction and are reflected back and forth at the surfaces. The diode lasers are relatively small and in many applications require a great deal of power in order to provide the lasing effect.
Many prior art applications employ feedback loops to control and monitor the amount of power applied to the laser in an attempt to obtain a more efficient and constant output and in an attempt to further reduce power requirements. Examples of suitable prior art can be made by referring to U.S. Pat. No. 3,633,124 entitled LASER WITH FEEDBACK CIRCUIT FOR CONTROLLING RELAXATION OSCILLATION issued on Jan. 4, 1972. Other patents such as U.S. Pat. No. 3,898,583 entitled LASER STABILIZING TECHNIQUE show the power control of a laser by means of feedback circuity. This and other patents as U.S. Pat. Nos. 3,946,335; 3,641,459 and 3,996,526 show various other techniques for limiting and controlling the amount of power applied to a laser.
As above indicated, an important application for the laser is the use of the same in a communications system to provide at an output, a light modulated signal. Hence, there are also various techniques shown in the prior art which employ circuitry to modulate the output of a laser. Examples of suitable techniques occur in some of the above noted references, while further references such as U.S. Pat. No. 3,925,735 entitled MODULATION APPARATUS WITH SEMICONDUCTOR DEVICE issued on Dec. 9, 1975 is indicative of one type of technique to control the modulation of a laser.
In any event, in most of these techniques, modulation is performed by applying a prebias signal to the laser. Modulation is then applied to the laser, having the prebias, by various means. By employing a prebias, one can obtain relatively high speed operation; which high speed operation is accomplished by relatively high power dissipation. Hence, it can be indicated that the speed of operation is proportional to the amount of power consumed.
A particularly widespread application for the semiconductor laser involves the transmission of coded information over fiber optic cables. In this application, the laser is modulated according to the information to be transmitted over a fiber optic cable. Due to the nature of the transmission characteristics of fiber optic cables, a DC balanced code is employed. This code is used in such transmissions as there is difficulty in transmitting DC levels over fiber optic cables. The use of such codes creates a problem in regard to a laser feedback system. The problem manifests itself in regard to the balanced code and if the code or the input signal to the laser becomes imbalanced, the feedback causes the drive current to the laser to increase. This effect can result in damage and destroy the laser. In such systems, as the light intensity increases due to the operation of the feedback circuit, the increase in drive current can cause facet damage to the laser crystal.
It is therefore an object of the present invention to provide apparatus which will operate in conjunction with a laser diode to protect the laser in the event of a duty cycle imbalance. This object is accomplished while providing modulation control of a semiconductor laser diode while eliminating a prebias of the laser and hence acheiving a substantial power advantage over the prior art devices.